Method of starting variable-speed induction motor

Abstract

This method of starting a variable-speed induction motor utilizes a speed control of the induction motor by varying the output frequency from a cycloconverter. The method comprises applying an output from the cycloconverter to the secondary winding of the induction motor while the primary winding of the induction motor is short-circuited during the start-up of the motor, applying a voltage lower than the line voltage to the primary winding when the induction motor is operating within an intermediate speed range, and applying the line voltage within a speed region near the rated speed. This method enables speed control over a wide range, and provides a sufficiently large start-up torque for the induction motor.

What is claimed is: 
     
       1. In a system consisting of a pump turbine, a variable-speed induction motor connected mechanically to the pump turbine to drive it, and a cycloconverter adapted to convert line frequency into any desired frequency so that the speed of said induction motor is controlled by varying a frequency output from said cycloconverter, a method of starting up said variable-speed induction motor comprising: applying the output from said cycloconverter to the secondary winding of said induction motor while the primary winding of said induction motor is short-circuited during the start-up of said induction motor; applying a voltage lower than the line voltage to said primary winding when said induction motor is operating at a speed within an intermediate speed range; and applying the line voltage to said primary winding when said induction motor is operating at a speed near its ranged speed. 
     
     
       2. A method of starting up a variable induction motor according to claim 1, wherein said intermediate speed range is between 50 to 100% of the rated speed.